1. Field of the Invention
The present invention relates generally to electron beam apparatus and electron microscopy methods.
2. Description of the Background Art
Optical microscopes, the simplest and most common instruments used to image objects too small for the naked eye to see, uses photons with visible wavelengths for imaging. A specimen is illuminated with a broad light beam, and a magnified image of the specimen can be observed using an eyepiece or camera. The maximum magnification of a light microscope can be more than 1000× with a diffraction-limited resolution limit of a few hundred nanometers. Improved spatial resolution in an optical microscope can be achieved when shorter wavelengths of light, such as ultraviolet wavelengths, are utilized for imaging.
An electron microscope is a different type of microscope. It uses electrons to illuminate the specimen and create a magnified image. The microscope has a greater resolving power than a light microscope, because it uses electrons that have wavelengths few orders of magnitude shorter than visible light. Electron microscopes can achieve magnifications exceeding 1,000,000×.
Scanning electron beam microscopes (SEMs), the most widely used electron microscopes, image the specimen surface by scanning it with a tightly focused high-energy beam of electrons in a raster scan pattern, pixel by pixel. In a typical SEM, an electron beam is emitted in a vacuum chamber from an electron gun equipped with a thermionic (tungsten, lanthanum hexaboride), thermally assisted (Schottky, zirconium oxide) or cold field emission cathode. The electron beam, which typically has an energy ranging from a few hundred eV to few tens keV, is collimated by one or more condenser lenses and then focused by the final objective lens to a spot about 1 nm to 100 nm in diameter. The beam is deflected by pairs of magnetic scanning coils or electrostatic deflector plates, sweeping in a raster fashion over a rectangular area of the specimen surface. Primary electrons can generate various signals from elastically scattered electrons, secondary electrons (due to inelastic scattering), characteristic Auger electrons and the emission of electromagnetic radiation. Each of the generated signals can be detected by specialized detectors, amplified and displayed on a CRT display or captured digitally, pixel by pixel on a computer.
Low energy emission microscopes (LEEM) and photoemission electron microscopes (PEEM) are projection (as opposed to scanning) electron microscopes, and thus resemble a conventional light microscope. In a LEEM, the electron gun forms a broad electron beam that is accelerated to typically 10 to 30 keV and passed through a beam separator, an energy-dispersive magnetic prism that separates the illumination and projection optics and bends the beam into the axis of a cathode objective lens containing the specimen. The objective lens is called a cathode lens as the specimen forms the negative electrode in this lens. A parallel flood beam then uniformly illuminates the specimen that is electrically biased at approximately the same potential as the cathode of the electron gun, so that illuminating electrons are decelerated in the objective lens, striking the specimen at energies typically between 0 to about 1000 eV. In the opposite direction, i.e. upward from the specimen, the objective lens simultaneously accelerates the scattered electrons and forms a magnified image of the specimen. As the electrons reenter the beam separator, they get deflected into the projection optics. The projection zoom optics forms an electron image on the scintillating screen that is then viewed by a CCD camera and further processed on a computer. The extremely low energy of the illuminating electrons makes LEEM an exquisitely sensitive surface imaging technique, capable of imaging single atomic layers with high contrast. The low landing energy of electrons is also critical for avoiding radiation damage, as high energy electrons in all keV kinetic energy ranges can cause unavoidable damage to many types of specimens.
Photoemission electron microscopes (PEEM) are projection electron microscopes, where the specimen is illuminated with UV photons or X-rays rather than electrons. Similar to a LEEM, the objective lens is a cathode lens with the specimen at a high negative bias. The photon flood beam uniformly illuminates the specimen, and the photoemitted electrons are accelerated by the objective lens and form a magnified image of the specimen.
One of the main drawbacks of conventional LEEM/PEEM is lateral resolution. In spite of the short deBroglie wavelength in the Angstrom range, the lateral resolution of conventional LEEM instruments is limited to a few nm and sub-nm resolution has not been achieved yet; and PEEM resolution ranges from 10 to 20 nm. The electron lenses used for imaging in a LEEM/PEEM, in particular the cathode objective lens, introduce spherical and chromatic aberrations that deteriorate the spatial resolution of a LEEM/PEEM. Effective means for improving the spatial resolution are therefore desirable if LEEM/PEEM instruments are to be used for imaging at higher spatial resolution.
Another drawback of a conventional LEEM/PEEM is its lack of energy-filtered imaging. The primary electrons scattered by the specimen produce electrons over a wide range of energies, from secondary electrons in the range of a few eV, to hundreds to thousands of eV for characteristic Auger electrons, and near the landing energy for elastically scattered electrons. X-ray photons result in the generation of photoemission electrons with a wide spectrum of energies, containing element-specific peaks that can be used to characterize the specimen. Electrons with different energies produce different image contrast and can provide comprehensive information about the specimen, including specimen topography, composition, crystalline structure as well as electrical and magnetic properties. In order to obtain detailed information about the chemical composition, interatomic bonding and local electronic states of non-periodic objects such as nanoparticles, interfaces, defects and macromolecules, an energy resolution of 0.1 eV or less is necessary to discern their characteristic electronic states. Effective means for selecting electrons emitted from the sample with a narrow range of energies for imaging as well as utilizing monochromatic illumination with an energy spread smaller than the desired energy resolution are therefore desirable for detailed characterization of specimens.
One approach to improve the spatial resolution and provide energy-filtering capability in a LEEM/PEEM is to use an aberration corrector based on an electron mirror, such as the one disclosed in U.S. Pat. No. 5,319,207, which is entitled “Imaging system for charged particles” and which issued Jun. 7, 1994 to inventors Harald Rose, Ralf Degenhardt and Dirk Preikszas. As shown in FIG. 1 a, this approach employs a dispersion-free magnetic beam separator and an electron mirror for aberration correction. The absence of energy dispersion after each deflection facilitates minimum combined aberrations between the energy dispersion and the chromatic and spherical aberrations of the electron mirror. However, this prior technique is disadvantageous in some aspects. The practical implementation of this approach is rather difficult, due to complexity of the magnetic beam separator. The dispersion-free beam separator is a rather complicated electron-optical element, consisting of a large number of coils with complex shapes that are difficult to construct and align. The machining, tight tolerances and assembly are challenging which makes tuning and alignment of the whole microscope difficult. In addition, the dispersion-free magnetic prism separator cannot be used for energy filtering, and an additional energy filter must be included in the projection optics which further complicates the microscope design, assembly and alignment.
Another approach to improve the spatial resolution and provide energy-filtering capability in a LEEM/PEEM is to use an aberration corrector based on an electron mirror, such as the one disclosed in U.S. Pat. No. 7,348,566, which is entitled “Aberration-correcting cathode lens microscopy instrument” and which issued Mar. 25, 2008 to inventor Rudolf Tromp. Unlike the prior technique disclosed in U.S. Pat. No. 5,319,207 that uses a complex dispersion-free beam separator, the apparatus and method disclosed in the U.S. Pat. No. 7,348,566 do not require the separator to be free of dispersion in order to achieve aberration correction. Instead, as shown in FIG. 1 b, it uses two dispersive magnetic beam separators of a practical design with simple square shaped coils that are much easier to machine, assemble and align. In addition, the technique disclosed in the U.S. Pat. No. 7,348,566 does not require an additional energy filter to carry out energy-filtered imaging. However, this technique using two beam separators is disadvantageous in some aspects. The additional deflection that transports the beam into the projection optics introduces energy dispersion that generates additional combination aberrations, including image tilt and off-axis astigmatism which can affect the image quality when sub-nm resolution is needed. In addition, the two beam separators have to be identical to ensure that the dispersion and all combination aberrations of the first prism separator are cancelled by the second prism separator, which may be difficult to achieve practically.
Another approach to improve the spatial resolution and provide energy-filtering capability in a LEEM/PEEM is to use an aberration corrector based on an electron mirror, such as the one disclosed in U.S. patent application Ser. No. 13/251,266, which is entitled “Compact arrangement for aberration correction of electron lenses” and which was filed Oct. 2, 2011 by inventor Marian Mankos.
Unlike the prior technique using a dispersion-free magnetic beam separator discussed above in relation to FIG. 1a, the apparatus and method disclosed in U.S. patent application Ser. No. 13/251,266 does not necessarily require a complex dispersion-free beam separator in order to achieve aberration correction. Instead, it uses a single energy-dispersive magnetic beam separator with practical design and simple square shaped coils that are much easier to machine, assemble and align.
Unlike the prior technique disclosed by Tromp in U.S. Pat. No. 7,348,566 using two energy-dispersive magnetic beam separators and discussed above in relation to FIG. 1b, the apparatus and method disclosed in U.S. patent application Ser. No. 13/251,266 does not require the use of an additional magnetic beam separator to achieve aberration correction. The prior technique disclosed by Tromp in U.S. Pat. No. 7,348,566 may result in additional combination aberrations in the projection optics including image tilt and off-axis astigmatism which can affect the final image quality. In addition, small differences in the geometry and excitation between the two energy-dispersive magnetic beam separators may result in incomplete cancellation of the dispersion combination aberrations, which may prevent the aberration corrector from fully correcting the objective lens aberrations. Instead, as shown in FIG. 1 c, it uses one dispersive magnetic beam separator and two electron mirrors to achieve aberration correction. The specimen is illuminated with an electron, UV-photon or X-ray beam, and a magnetic beam separator deflects the electron beam emitted from the specimen and magnified by a cathode objective lens towards a first electron mirror. The magnetic beam separator introduces an angular dispersion that disperses the incoming electron beam according to its energy. An electron lens is configured to focus the dispersed electron beam at the reflection surface of a first electron mirror and introduce symmetry so that the reflected electron beam passes through the magnetic beam separator a second time and exits without energy dispersion. The electron beam then enters a second electron mirror that is configured to correct for one or more aberrations of the cathode objective lens and reflect the electron beam back into the magnetic beam separator. After a third deflection through the magnetic beam separator the electron beam is transported into the projection optics and magnified on a viewing screen.
Another important drawback of a conventional LEEM/PEEM is its lack of imaging capability for insulating samples. When a conventional LEEM instrument is used to image insulating specimens, the low landing energy exacerbates charging effects resulting in significantly reduced image quality. The imbalance between the arriving and leaving flux of electrons causes the surface to charge up, resulting in increased blur and distortions. In many cases, the built-up surface charge can rapidly discharge in an arc, resulting in specimen damage. On a homogeneous insulator surface, the charging can be suppressed by operating at a landing energy resulting in a net electron yield of 1. However, this approach restricts the landing energy and typically does not work when different insulating materials are present on the surface. Effective means for controlling local surface charging are therefore desirable if LEEM instruments are to be used for imaging of insulating samples. None of the above mentioned aberration correction inventions shown in FIG. 1 a-c have a provision to mitigate the charging effects.
One possible approach that can be used to solve the charging problem in a LEEM/PEEM is the dual illumination beam approach. In a LEEM with dual-beam illumination, two electron beams with different landing energies are used to mitigate the charging effect. The low-energy electron beam with landing energy near 0 eV is partially mirrored and partially absorbed, charging the surface negatively. The high-energy electron beam (˜100 eV or more) emits secondary electrons with an electron yield that exceeds 1, charging the surface positively. However, when two beams with opposite charging characteristics, i.e. a low-energy mirror electron beam and a high-energy electron beam are superimposed on the specimen, charging effects can be neutralized. The challenge is to devise an electron optical system that can deliver overlapping illumination of the low-energy mirror and high-energy electron beams at preferably normal incidence on the specimen, i.e. a system that combines two parallel electron beams with different energies and beam currents at the specimen surface.
One approach to combine two illuminating electron beams with different charging characteristics to mitigate the deleterious charging effects is disclosed in U.S. Pat. No. 6,803,572, which is entitled “Apparatus and methods for secondary electron emission with a dual beam” and which issued Oct. 12, 2004 to inventors Lee H. Veneklasen and David L. Adler. As shown in FIG. 1 d, this approach employs two co-planar guns with different beam energies and inclined beam axes that generate two illumination beams. The guns are configured such that the angle of inclination is equal to the difference in bending angles caused by the magnetic prism separator. However, this prior technique is disadvantageous in some aspects. The practical implementation of this approach is rather difficult, due to the small difference in deflection angles. For example, for a 30 keV electron beam energy and beam energy differential of 300 eV, the difference in deflection angles amounts to only about 5 mrad, i.e. about ⅓ of a degree. This means that the guns must be impractically far from the prism in order not to overlap. In principle, one can increase the angular separation by biasing a drift tube in the beam separator at high negative potential and thus lowering the beam energy while electrons pass through the beam separator. However this is not desirable due to increased Coulomb interactions and geometric aberrations that deteriorate the spatial resolution. In addition, it complicates the design and increases the likelihood of high-voltage arcing.
Another approach to combine two illuminating electron beams with different charging characteristics to mitigate the deleterious charging effects is disclosed in U.S. Pat. No. 6,803,571, which is entitled “Method and Apparatus for Dual-Energy E-Beam Inspector” and which issued Oct. 12, 2004 to inventors Marian Mankos and David L. Adler, is shown in FIG. 1 e. Unlike the prior technique using two inclined beams discussed above, the apparatus and method disclosed in U.S. Pat. No. 6,803,571 do not necessarily require biasing of the separator at high voltage in order to achieve sufficient angular separation of the low and high energy beams. In addition, the presently disclosed technique does not require two electron guns to be in close proximity to each other. The apparatus includes a dual-beam electron gun that is configured to generate both a high-energy electron beam component and a low-energy electron beam component. In one implementation, the dual-beam electron gun is composed of two concentric cathodes, an inner disc and an outer annulus. The inner disc may be biased at a high negative voltage with respect to the specimen, while the outer annulus may be biased by an additional negative voltage with respect to the inner disc. However, this prior technique using a dual-beam electron gun is disadvantageous in some aspects. The proximity of the two cathodes in the gun at different temperatures and potentials results in complex crosstalk effects, beam current drift and long settling times, which makes it difficult for practical use and may reduce stability and reliability of the electron beam apparatus. These issues can be resolved when an electron mirror and prism are used to recombine two spatially separate electron beams.
Another approach to combine two illuminating electron beams with different charging characteristics to mitigate the deleterious charging effects is disclosed in U.S. Pat. No. 7,217,924, which is entitled “Holey mirror arrangement for dual-energy e-beam inspector” and which issued May 15, 2007 to inventors Marian Mankos and Eric Munro. As shown in FIG. 1 f, the apparatus includes a illumination configuration with two perpendicular branches which are connected by a magnetic prism beam combiner. The first branch includes a first electron gun at a first (lower) energy, and the second branch includes a second electron gun or source at a second potential energy. The second branch also includes a semitransparent electron mirror that reflects the lower energy beam and transmits the higher energy beam. This prior technique allows the use of two conventional single beam guns, which simplifies the gun design and makes the operation more reliable. However, this prior technique is disadvantageous in some aspects. The column requires an additional bending element, i.e. a magnetic prism array, and complex transfer optics to assure cancellation of the dispersion.
Another approach to combine two illuminating electron beams with different charging characteristics to mitigate the deleterious charging effects is disclosed in U.S. Pat. No. 8,258,474, which is entitled “Compact arrangement for dual-beam low energy electron microscope” and which issued Sep. 4, 2012 to inventor Marian Mankos. As shown in FIG. 1 g, a first electron beam source is configured to generate a low-energy electron beam, and an energy-dispersive device deflects the low-energy electron beam towards an Einzel lens that acts as an semitransparent electron mirror. The Einzel lens is biased to reflect the low-energy electron beam. A second electron beam source is configured to generate a high-energy electron beam that passes through an opening in the Einzel lens. Both the low- and high-energy electron beams enter the same energy-dispersive device that deflects both beams towards the specimen. A deflection system positioned between the high-energy electron source and Einzel lens is configured to deflect the high-energy electron beam by an angle that compensates for the difference in bending angles between the lower- and higher energy electron beams introduced by the energy-dispersive device, therein allowing both the lower- and high-energy beams to strike the specimen at normal incidence.
However, none of the above-mentioned prior dual-beam beam approaches allow for aberration correction, monochromatic illumination and energy filtered imaging. In addition, none of the above-mentioned aberration-correction and energy-filtering approaches allow for dual beam illumination. An improved LEEM/PEEM apparatus and methods for providing simultaneous aberration correction, monochromatic illumination, energy filtering and dual beam illumination are desirable.